1. Field of the Invention
This invention relates to improvements in mass storage devices, or the like, and more particularly to improvements in detectors and methods for detecting and processing servo information contained on a data track of a mass data storage device, or the like, and still more particularly to improvements in such Gray code servo signal detectors and techniques which employ PR4 equalization techniques and inverted non-return-to-zero (NZRI) Gray code encoding.
2. Relevant Background
In modern computers and computer-type applications, one or more mass data storage devices may be employed. Typical mass data storage devices, often referred to as hard disk drives, CD-ROMs, or the like, have one or more rotating data storage disks. The data storage disks may have thereon, for example, a magnetic, optical, or other media that can contain data. In such devices, data is generally recorded in certain field portions of rings or tracks that are physically located progressively radially outwardly from the center of the disk.
The term "data" is used herein generally to mean data of all kinds, including servo data, such as Gray code information, AGC signals, head alignment bursts, and the like, recorded in servo sectors, and including user data, recorded in user data sectors, as described below in detail. Of particular concern herein, each track has one or more servo sectors located at spaced locations along the track. Each servo sector has a number of fields, each for providing information for location or control of the head data transducer. Typically, for example, each servo sector includes a field that contains an AGC burst, which, when read, enables AGC circuitry associated with the disk to automatically adjust the gain in the head amplifiers to enable the following data to be properly detected.
Generally, following the AGC burst is a field that contains one or more sync marks so that the longitudinal position of the head relative to the track of interest can be determined. It should be noted that the field that contains the sync marks might not follow the AGC field in every embodiment, but may be located at some other place along the length of the track. The sync marks may be used, for example, to enable subsequent fields, such as the user data sectors or Gray code data to be located by counting a predetermined elapsed time from the time that the sync marks are detected.
A Gray code field may follow the sync mark field in the servo sector. The Gray code field may contain Gray code data from which the identification of the particular radial track over which the head is positioned can be established. Following the Gray code field is a field containing binary data, for example, to contain longitudinal track identification information, so that the identity of each track region between adjacent servo sectors can be established. After the binary data field, a number, typically four, burst fields are presented for more precision alignment of the head laterally with respect to the selected track.
In order to read the data that has been previously recorded on the data medium one or more data transducers, or heads, are provided that are selectively radially moved over a desired ring containing the data that is to be read. The aforementioned Gray codes pre-recorded onto each data ring are decoded to determine the instantaneous position of the data transducer heads, in known manner. The data transducer heads are typically positioned by means of a closed-loop servo system in accordance with the decoded Gray code that has been detected. More particularly, the data transducer heads read the Gray code servo information recorded within data tracks on disks. The servo information typically includes track addresses, and optionally sector addresses and servo bursts. The track addresses are used as coarse positioning information and servo bursts are used as fine positioning information.
As the transducer heads are being moved to a desired track location, the transducer head reads the track addresses provided by the Gray codes in order to determine its instantaneous location. Of course, the transducer head may be positioned between two adjacent tracks, and may receive a superposition of signals from both tracks; however, due to the data characteristics of Gray codes, the position ambiguity can be easily resolved. Thus, when the head is on an interface between two tracks, either of the two track addresses will be correctly detected, due to the characteristics of the Gray code used.
The data sectors on the selected track may be synchronously recovered after timing acquisition by a phase lock loop circuit, but the detection of the servo sectors on a track are usually asynchronously performed.
It is difficult to realize high-speed detection and high-density recording by asynchronous servo detection methods. Consequently, various synchronous servo techniques have been employed, one of which being PRML signal processing. In this approach, timing is synchronized in the servo preamble region by a phase lock loop circuit, and the track address and servo bursts are synchronously sampled and are decoded.
Recently, disk drive manufacturers have been striving to achieve greater capacity in the disk drives that they have been producing. To this end, data has been recorded onto the data medium more densely, and other techniques have been employed to realize this goal. As result, interference between adjacent data symbols often referred to as inter-symbol-interference, or ISI, has increased, lowering the signal-to-noise ratio in the detected signals from the data medium. As a result, it has become more difficult to properly detect the signals read from the data medium, which, in turn, has resulted in increasing the difficulty in rapidly and properly positioning the data head transducers.
In the past, many manufacturers have used a rate 1/3 Gray code and a PR4 Viterbi detector for Gray code detection. This technique has been preferred because the signals of Gray codes are typically equalized to PR4 targets, and the PR4 Viterbi detector typically used can realize the performance of the 1/3 Gray code, which has an Euclidean distance of d.sup.2 =2, which results in about a 3 dB improvement in the signal-to-noise ratio. Although proposals have been made to use 1/4 rate Gray codes, which have increased performance, and which have Euclidean distances of d.sup.2 =4, the 1/4 Gray code signals are usually equalized to a PR4 Viterbi detector. However, the use of PR4 signals and the PR4 Viterbi detector can not realize the performance of the 1/4 Gray code because the signal-to-noise ratio improvement realized by the use of a PR4 Viterbi detector is limited to d.sup.2 =2, or 3 dB.
If an EPR4 Viterbi is used, a Gray code signal equalized by a PR4 equalizer is required, and an additional (1+D) filter is needed between the equalizer and the Viterbi. The (1+D) filter, however, increases the noise in the channel. Therefore, the overall performance improvement is less than 3 dB in spite of the increase of the Euclidean distance (d.sup.2) in the code. Thus, the EPR4 channel with a 1/4 Gray code is only about 1.5 dB better then a PR4 channel with a 1/3 Gray code in high channel densities (k=PW50/T.sub.c, more than 2.5), but the EPR4 channel with the 1/4 Gray code is worse than the PR4 channel with the 1/3 Gray code in the low channel densities (k&lt;1.5) in.
It is possible to improve the PR4 and EPR4 Viterbi detector performance by using an additional error correction unit (ECU) which corrects the code violated data. But the improvement is not significantly large.
What is needed, therefore, is a system and method that reduces the signal-to-noise ratio in the data read channel of a mass storage device, or the like, and in particular to circuitry that can be used to detect and employ 1/4 Gray code signals together with a PR4 Equalizer and a matched filter detector in the positioning of the data head transducers.